Are you looking to convert a selection of songs into a single continuous length song or are you looking to DJ with music as it plays?

For gapless playback where the start of the next song plays on top of the end of the current song, have a look in your music playback software to see if it already has this feature. For example, in Winamp, you can enable this by going into its Preferences, click "Output plug-in" on the left and then the "Configure" button. Go into the "Other" tab and enable the "Silence remover".

To DJ with songs as if they are 45RPM records on a turntable, the DJ software Ots Labs (website) does a pretty good job at this. The bedroom edition is free for home use, however the pro edition which includes the abilty to record your mixes is priced at about $80.